The so-called tandem type image forming apparatus having photosensitive drums corresponding to yellow, magenta, cyan and black respectively arranged in line in a transport direction for sheets is known in relation to an image forming apparatus such as a color printer.
In the tandem type image forming apparatus, a developing roller and a transfer roller are provided correspondingly to each photosensitive drum. A developing bias for forming potential difference between the developing roller and the photosensitive drum is applied to the developing roller. A transfer bias for forming potential difference between the transfer roller and the photosensitive drum is applied to the transfer roller.
In image formation, an electrostatic latent image responsive to an image to be formed on each sheet is formed on the photosensitive drum. When the electrostatic latent image is opposed to the developing roller upon rotation of the photosensitive drum, a toner is fed from the developing roller to the electrostatic latent image due to the potential difference between the photosensitive drum and the developing roller. Thus, the electrostatic latent image is developed into a toner image, and this toner image is carried on the photosensitive drum. The transfer roller is arranged on a position opposed to the photosensitive drum through a transport passage for the sheet. When the toner image carried on the photosensitive drum is opposed to the transfer roller through the sheet upon rotation of the photosensitive drum and transportation of the sheet, the toner image is transferred from the photosensitive drum to the sheet due to the potential difference between the photosensitive drum and the sheet. In formation of a color image on the sheet, toner images carried on the photosensitive drums are transferred to the same position of the sheet. The color toner images overlapped on the sheet are fixed to the sheet as a color image by heating and pressurization.
In relation to such tandem type image forming apparatus, there are image forming apparatuses having a single-side printing mode of forming an image on a single surface of the sheet and a double-side printing mode of forming images on both surfaces of the sheet. In the double-side printing mode, an image is formed on a first surface of the sheet, the sheet is thereafter transported in a reversed manner, and another image is formed on a second surface of the sheet opposite to the first surface.
The toner constituting the toner image formed on the sheet is charged in the same polarity as the surface of the photosensitive drum. Therefore, the charge quantity of the toner on the sheet is increased (the toner is charged up) every time the sheet is opposed to the photosensitive drum. Consequently, the surface potential (the charge quantity) of the sheet transported while being successively opposed to the photosensitive drums is increased along with the progress of the transportation. Particularly in the double-side printing mode, the sheet is dried due to the heating for fixing the toner image in formation of the image on the first surface of the sheet, and the electrical resistance of the sheet is increased following this. In formation of the image on the second surface of the sheet, therefore, the surface potential of the sheet is remarkably increased and a leakage of transfer current (electricity fed to the transfer roller) to a portion of the surface of the photosensitive drum not in contact with the sheet is increased as the transportation of the sheet progresses. When the surface potential of the sheet is increased, the potential difference formed between the photosensitive drum and the sheet is reduced, and hence the transfer efficiency of the toner from the photosensitive drum to the sheet is reduced. Consequently, the quality of the color image formed on the sheet is reduced.
In order to prevent this, the transfer bias applied to each transfer roller may conceivably be set up in consideration of the increase in the potential of the sheet. In this case, the transfer bias applied to the transfer roller on the most downstream side in the transport direction for the sheet is set up to the greatest value among the transfer biases applied to the four transfer rollers. In the double-side printing mode, further, the transfer bias applied to each transfer roller in formation of the image on the second surface of the sheet is set up to a value greater than the transfer bias applied to each transfer roller in formation of the image on the first surface of the sheet.
According to this technique, however, the potential difference between the transfer roller on the most downstream side in the transport direction for the sheet and the sheet may be excessively increased to cause discharge between the transfer roller and the sheet in formation of the image on the second surface of the sheet.